Electro-pneumatic converters, such as current to pressure transducers are in common use as field instruments mounted in pipeline systems for controlling the process fluid. Subsequently these devices are installed in potentially hazardous explosive environments. Such devices receive, for example, a variable current input signal of between 4-20 mA and eventually provide a variable pressure output to an actuator for a fluid control valve. Since these devices can be employed in a potentially explosive environment, to provide an explosion proof device the electrical and pneumatic components are isolated within an explosion proof portion of the transducer/positioner unit, except for the pressure gauges which are normally located on the unit exterior.
With presently available electro-pneumatic converters operating in a potentially explosively hazardous area, in the event either service of the unit or normal maintenance is required, the electric power must be disconnected and/or the entire unit must be removed from the potentially hazardous area in order to be worked on. Occasionally, for instance, the pneumatic elements must be adjusted or removed and replaced. In present units, if the seal of the explosion proof portion of the unit is removed in order to get access to these pneumatic elements, then a potentially unsafe condition is created where any spark caused in the electrical elements could ignite potentially explosive gases. Accordingly, shutting down the electrical power in order to service or remove the unit from the hazardous area is time consuming, costly, and wasteful.
In addition, with the electrical and pneumatic components maintained within an explosion proof portion of the transducer/positioner unit, the mounting of the pressure gauges on the portion outside the explosion proof portion of the unit is required. However, this exposes the pressure gauges to the atmosphere as well as to physical damage from unintentional blows to the pressure gauges protruding even slightly from the transducer/positioner exterior surface.
Accordingly, it is desired to provide an electro-pneumatic converter which can be used in a potentially explosive environment and wherein pneumatic components can be serviced without requiring electrical shutdown or removal of the entire unit from the hazardous area.
In addition, it is desired to provide an electro-pneumatic converter where the pressure gauges can be protected from the atmosphere as well as from any unintentional physical damage.
Currently available pressure transducer instruments contain a pressure to current sensor to convert a pressure signal to a current signal in supplying feedback to the instrument. Analog pressure transducer units are available, as well as respective digital pressure transducer units. Also currently available are separate valve positioner units which incorporate feedback from the valve supplied from a mechanical linkage with the valve stem. Both analog valve positioner units and digital valve controller units are separately available.
Typically, the customer chooses the type of instrument needed in order to fit within his present system. Thus, a customer may initially choose to purchase a pressure transducer instrument in analog form as most conveniently adaptable to his present system. If the customer's system changes or he wishes to modify his system to operate in digital form, the customer must then purchase the required separate instruments. Also, if the customer wants to change to a valve positioner configuration when he initially purchased a pressure transducer instrument, he must purchase a new valve positioner device in the proper data format to fit his changed system.
It is therefore further desired to provide an electro-pneumatic instrument which is usable and readily convertible from a pressure transducer to a valve positioner or vice versa. In addition, it is desired to enable a user to readily convert from an analog data handling capability to a digital data handling capability or vice versa. Furthermore, it is desired to enable the instrument user to incorporate and to change to any desired communications data protocol.
In existing electro-pneumatic instruments, the instrument housing is usually formed of a casting. The casting must then be drilled with precise holes to form passageways and interconnected passageways to enable the desired communication of fluid between components. Forming of the desired passageways by drilling intersecting holes in the casting requires time consuming precision drilling and set up of the housings for drilling. Present instruments also utilize many individual sub-assembly components requiring stocking and assembly time. The instrument covers, for instance, normally require two or more pins and locks or other multi-part fasteners to mount the cover to the instrument. This requires an inventory of the several parts and an inordinate amount of assembly time to assemble the cover to the instrument.
Pneumatic relays, used extensively in positioners and transducers, normally have been made from aluminum based materials. Machine screws are used to assemble the aluminum relay body components together while clamping rubber diaphragms and O-rings to provide the pressure seals. Assembly of these numerous subassembly components of present pneumatic relays is tedious and costly in the manufacturing environment.
It is desired therefore to eliminate components or at least reduce the number of components required for an electro-pneumatic instrument.